A party of rafters from the National Park Service, Moffat County and Yampa River State Park are dwarfed by the landscape of Yampa Canyon, 600 feet below in Dinosaur National Monument. The photograph was taken from Wagon Wheel Point.

Editor's note: This story was modified on July 15 to clarify that the Yampa River is not un-dammed, but the annual pattern of peak flows during spring snowmelt remains intact.

Deep in Dinosaur National Monument on a broad sandbar in the Yampa River, a grisly scene is being carried out. It’s a horror story so cruel, it might make even movie director Wes Craven cringe.

A long black wasp is struggling to drag a bulbous, beige spider four times its body mass across the sand. The spider still is alive, but it puts up almost no resistance.

The wasp is commonly known as a tarantula hawk because in southern Arizona, its cousins commonly take down tarantula spiders. Scientists have determined that the wasps deliver the most painful sting of any insect. Their purpose, however, is merely to paralyze their victims so they can feed them to their as-yet unborn offspring.

The action scene that played out between the wasp and the ill-fated spider provided just one of many natural history lessons during a whitewater rafting trip down Yampa Canyon in June. Once compiled, those lessons about rare plant and animal communities help to define an endangered ecosystem.

The float trip was part of the Yampa River Awareness Project and was hosted by the national conservation organization American Rivers, Friends of the Yampa and OARS river outfitters. The intent was to bring attention to the Yampa as the last free-flowing tributary of the Colorado River.

If one includes three reservoirs on the headwaters above the town of Yampa, where the stream is known as Bear River, and two more reservoirs above Steamboat Springs, there are five dams on the upper Yampa. The river is not un-dammed, but for now, the Yampa mirrors its historic flows.

American Rivers Senior Communications Director Amy Kober posed the question: “What is the value of the Yampa?”

“On the Yampa, you see the river as it should be,” Kober said. “At every scale, there is something interesting going on. For endangered fish like the Colorado pikeminnow, the Yampa may be our best chance to save species that are thousands of years old from being lost forever.”

Expedition videographer and former longtime river guide Michael Bye, of Steamboat Springs, spoke about the values that humans draw from wild rivers.

“If you can go down this river, you can shake the outside world off” if only temporarily, Bye said. “The difference between Yampa Canyon and other river trips is that it is wilderness, and there are almost no signs of civilization. So many other rivers have railroad tracks and roads” running alongside of them.

Members of the expedition that took place just after high water June 7 to 11 included conservationists, water policy makers and scientists from the Colorado Water Conservation Board, The Nature Conservancy, Colorado Water Trust, the Upper Yampa Water Conservancy District and the Colorado River Water Conservation District.

Life and death on the river

The tarantula hawk occupies a rare environment along the Yampa, widely regarded as the last of the wild desert rivers that once flowed unrestrained through the canyons of the Colorado Plateau. Just a couple of weeks before the little drama unfolded on the sandbar, the area would have been covered in the spring snowmelt. The spider sought to procreate as soon as the flood began to subside. Up and down the canyon, other species, both plants and animals, were doing the same.

Before going out to hunt spiders in the early morning, the wasp dug a small pit in the sand and may have lined its rim with tiny pebbles and sticks so that it could identify it upon its return.

John Saunders, a longtime outdoor education professor at Colorado Mountain College in Steamboat Springs, described what the tarantula hawk had in store for the spider.

“She will put the spider in the hole and lay a single egg on its abdomen,” Saunders said. “When the (larva) hatches, it will slowly eat the spider” until it is ready to build a cocoon.

But here’s the really spooky part of the spider hawk’s story: Somehow, the wasp’s larva knows not to eat the spider’s heart until the very end, lest the arachnid die and go stale too soon.

Our group, by tromping across the sandbar, might have obliterated the tarantula hawk’s excavated hole, for we never found it. But the spider’s fate already was sealed.

The fate of the Yampa River itself was the topic of discussion on the Yampa River awareness trip.

What makes the Yampa special is the fact it is largely undammed, allowing it to behave the way it has for millennia. But it’s that same fact that suggests the river will become a target for water interests from Colorado’s Eastern Plains to Nevada. More and more, water users are looking at the Yampa as human demand for water to build cities, extract energy and feed the world in an era of climate change has begun to exceed supply.

Before last month’s expedition was over, its members would encounter numerous desert bighorn sheep, once absent for many years from their natural habitat. A shy, little rattlesnake hid beneath a twisted juniper log right where rafters eddied out to scout the nastiest rapid in the canyon. Elsewhere, mating Woodhouse toads took advantage of a warm pool left behind by the receding river to leave strands of eggs. And the root systems of wildflowers like the primrose slurped water from beneath the sandbars and beckoned to nocturnal moths to pollinate them.

As gruesome as the tale of the tarantula hawk is, it’s a part of the natural order of things on the desert reaches of the Yampa, where for thousands of years many species of plants and animals have been able to count on the river’s extreme behavior.

It is the abrupt rise and fall of the river — the steep climb to this peak and the steep drop off the backside that coincides with the season when nature is renewing itself — around which the plants and animals of the canyon have evolved.

Hydrologists who plot the seasonal flows of the Yampa on two axes call the annual rhythm of the river its hydrograph. And from the Colorado pikeminnow sitting atop the river’s natural food chain to the tarantula hawk and the delicate primrose that bloom in the exposed cobble bars in June, they all are married to the hydrograph.